1. Technical Field
The present invention relates to a controller and a method for determining a failure of a thermostat provided in a cooling system of an internal-combustion engine.
2. Description of the Related Art
A vehicle comprises a radiator that supplies cooling water for cooling an internal combustion engine. The internal combustion engine and the radiator are connected via a passage, in which a thermostat is provided for opening and closing the passage. The thermostat includes a valve that is driven to open and close according to a temperature of cooling water. The thermostat closes when the temperature of cooling water is low, so that the cooling water circulates within the engine. The thermostat opens when a temperature of cooling water is high, so that the cooling water circulates between the radiator and the engine.
There are typically two types of thermostat failures. One is an opening failure where the thermostat is kept open and is unable to close. The other is a closing failure where the thermostat is kept closed and is unable to open. If the opening failure occurs when the engine in a cooled state is started, the cooling water from the radiator circulates through the engine, thus interfering with a rise in the water temperature of the engine. If the closing failure occurs, the water temperature of the engine continues to rise and may exceed a predetermined temperature because the cooling water does not circulate between the engine and the radiator. In the closing failure, the engine may overheat.
When a failure occurs in the thermostat, it is desirable to detect the failure quickly and give a driver a warning of the failure. In case of the closing failure, the driver can recognize its occurrence because a water temperature meter provided in a display panel of the vehicle indicates a rapid increase. On the other hand, in case of the opening failure, the driver cannot promptly recognize it.
A method for detecting the opening failure of a thermostat is disclosed in Japanese Patent Application Unexamined Publication (Kokai) No. 2000-8853, which is assigned to the same assignee of the present invention. According to the method, if a detected water temperature has not reached a predetermined normal determination value when an estimated water temperature, which is determined according to driving conditions of the engine, has reached a predetermined failure determination value, it is determined that the opening failure has occurred in the thermostat. In other words, if a rise in the detected water temperature is less than a rise in the estimated water temperature, it is determined that the opening failure has occurred in the thermostat.
An engine has a characteristic that a water pressure of the cooling water that circulates through the engine increases in proportion to a rotational speed of the engine. When the engine operates at a higher rotational speed, the water pressure may exceed a set pressure which is specific to the thermostat.
If the water pressure exceeds the set pressure, the thermostat opens irrespective of the water temperature of the engine. The opening of the thermostat may cause a situation in which the water temperature of the engine does not rise. In this situation, the above conventional method may determine that a failure has occurred in the thermostat because the detected water temperature of the engine does not reach the normal determination value. This determination is wrong because the opening operation of the thermostat caused by a high rotational speed of the engine is normal.
Thus, there is a need for a controller and a method for prohibiting a determination of a failure of the thermostat when the thermostat is opened due to a high rotational speed of the engine.
According to one aspect of the invention, a controller determines a failure of a thermostat that opens and closes a passage through which cooling water circulates between a radiator and an engine. The controller prohibits the failure determination if a water temperature of the engine decreases by more than a predetermined amount during a predetermined period from the time when a rotational speed of the engine exceeds a predetermined rotational speed.
In accordance with the invention, when the thermostat opens due to a high rotational speed of the engine, the failure determination is prohibited. Therefore, it is avoided that the thermostat operating normally is judged to be faulty.
According to one embodiment of the invention, the controller further updates a maximum value of the water temperature of the engine during the predetermined period. The amount of the decrease in the water temperature is measured with respect to the maximum value. Thus, it can be easily determined whether the thermostat has opened due to a high rotational speed of the engine. The failure determination is prohibited at an appropriate timing.
According to another embodiment of the invention, the decrease of the water temperature of the engine is measured with respect to a water temperature detected in a previous cycle. Thus, the failure determination is prohibited when the thermostat is opened in the current cycle due to a high rotational speed of the engine.
According to another aspect of the invention, the controller carries out the thermostat failure determination process when all of the following conditions are met:
a) an outside air temperature at the time of starting the engine and the water temperature of the engine at the time of starting the engine are within predetermined ranges, respectively;
b) a difference between the outside air temperature at the time of starting the engine and the water temperature of the engine at the time of starting the engine is equal to or less than a predetermined value; and
c) the outside air temperature does not decrease by more than a predetermined amount from the time of starting the engine. Thus, the thermostat failure determination is carried out under circumstances where the engine is sufficiently soaked and a variation in the outside air temperature is small.
According to another aspect of the invention, an estimated water temperature is determined. A rise in the water temperature of the engine detected by a sensor is compared with a rise in the estimated water temperature. The controller determines whether a failure has occurred in the thermostat based on the comparison result. By using the estimated water temperature, it can be determined whether a rise in the detected water temperature is appropriate.
According to one embodiment of the invention, the estimated water temperature is determined based on the water temperature of the engine that is detected at the time of starting the engine and a thermal load that contributes to an increase in the water temperature of the engine. Thus, the accuracy of the failure determination is improved.